Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 58-200423 (U.S. Pat. No. 4,511,484) discloses kneading of a magnetic coating solution compound incorporating a magnetic metal powder in which heat generation from the metal powder is suppressed by keeping the kneading atmosphere oxygen concentration at not more than 11%, preferably at not more than 7%, specifically at 2% by N.sub.2 gas purge.
U.S. Pat. No. 5,061,516 discloses prevention of ferromagnetic metal powder oxidation by lowering the oxygen concentration to not more than 5%.
Although it is possible to avoid the danger from firing of metal powder and prevent the deterioration of electromagnetic transferring property due to oxidation by lowering the oxygen concentration below 5%, it results in seasonal variation in kneading results and hence in electromagnetic transferring property, necessitating a means of stabilizing the state of kneading.